Police are investigating a report of an attempted sexual assault and armed robbery Saturday night.

At about 11:45 p.m., officers responded to Lazazzero Playground, at Lake Street and Shore Road, to a report of a man trying to attack a woman, police said.

The woman told officers she was restocking the snack shack, police said, when a man wearing a grayish-tan mask walked in and threatened her with a black-handled, black-bladed knife demanding she take off her clothes.

Police said the woman pulled the fire alarm and was able to run away. The man also ran away eastbound parallel to Lake Street, police said.

The man is described as possibly being Hispanic, 5 feet 10 inches in height, and at the time was wearing gray sweatpants and a gray shirt.

Anyone who was in the area or has any information regarding this incident is urged to call Waltham Police detectives at 781-314-3550 or the Anonymous Tip Line at 781-314-3636, or submit an anonymous tip using the Citizen Observer App on the department’s Facebook page, www.facebook.com/WalthamMAPolice.